This is continuation of my last note about building a TL494 modified sine wave inverter by modifying TL494 PWM controller based square wave inverter. In my last note, I wrote about the square wave-based push pull TL494 inverter. I wrote about the circuit and general idea of converting it to pure sine wave inverter. But as already said in that note, it is complex and tedious and requires replacements of components as well as using new chips. I wanted to modify with minimum replacements and complexity. This led me to try to build not pure sine inverter but a modified sine wave inverter. It is of course not as efficient as the pure sine wave inverter but still is more efficient than square wave TL494 inverter.
In that earlier note, I also said that there are several options to build a modified sine wave inverter. One of the main choices is the transformer choice. We can either use a large iron-core transformer or small ferrite-core transformer for inverter design. Since the inverter circuit that I wanted to modify used ferrite core transformer, I started to design based on that. I also wrote about Large Iron-Core transformer Vs Small Ferrite-Core transformer for Inverter.
Continuing from there I will now write about how to design modified sine wave inverter with ferrite core transformer. To understand the modification required and how the overall circuit works, the circuits diagram of the previous TL494 based push pull inverter is shown below.
This inverter converts 12V DC to 220V AC by chopping the high voltage generated by the center tapped transformer into high voltage pulses which appears at the output. That chopping is done by the four IRFZ44N N-channel MOSFET. The TL494 IC is configured to oscillate and generate 50% duty cycle square wave signal at high frequency(>20KHz). So, the output was pulsed square wave signal with frequency greater than 20KHz and RMS magnitude of around 220V. These inverter designs are common in very cheap or specific-purpose inverters (like those used for old "Electronic Transformers" or some CCFL drivers). It can be used or is intended for "Resistive Loads" only, such as incandescent light bulbs or some specific switching power supplies (like laptop chargers) that can accept high-frequency AC. But the circuit cannot be used and should not be used for other home electrical appliances. It can destroy them.
So, we need to modify so that it can be used as more general-purpose inverter. To do this we need to add another 2nd stage. The 2nd stage is necessary because a single TL494 IC cannot simultaneously manage the massive voltage boost and the low-frequency oscillation required for a 220V AC output. The first stage square wave inverter is also required as it operates at 30kHz to 50kHz high frequency and this high speed is mandatory to make the transformer small and efficient.
So, we need to build whole new 2nd stage as well as make minor modifications to the 1st stage. These modifications are described below.
1st stage of TL494 Modified Sine Wave Inverter Design
We have to modify the followings to convert the above shown square wave TL494 inverter to 1st stage of modified sine wave inverter.
1. Feedback for voltage regulation
We have to add feedback to the circuit for voltage regulation. The reason is, with modified sine wave inverters, one major issue is that the output voltage drops significantly under load. To remedy this, we can use the TL494's internal Error Amplifiers to stabilize this. This involves sampling the output voltage (through a small sensing transformer or a high-value resistor divider). The sampled voltage is feed this into Pin 1 (1IN+). The Pin 2 (1IN-) is used as your reference (connected to a divider from the 5V Ref). This then allows the TL494 to automatically widen the pulses if the battery voltage drops or the load increases, keeping the RMS voltage stable.
In the current schematic above, Pin 2 and Pin 15 are tied directly to Pin 14 (5V). We need to "free" them to act as a proper comparison point.
Step-by-Step Feedback Implementation
a. Setup the Reference Voltage (Pin 2)
Currently, Pin 2 is at 5V because it is connected to pin 14 which is a bit high for a reference. It’s better to set it to 2.5V so the chip has more "room" to adjust.
The Change: Disconnect Pin 2 from Pin 14.
The Divider: Connect a 10kΩ resistor from Pin 14 (5V) to Pin 2, and another 10kΩ resistor from Pin 2 to Ground (Pin 7).
Result: Pin 2 now sits at a stable 2.5V reference.
b. Sample the High Output Voltage (Pin 1)
We need to scale the 310V DC down to match that 2.5V reference.
The Divider: Connect a 1.2MΩ resistor (or two 600kΩ in series for safety) from the Positive High-Voltage DC Bus to Pin 1.
The Ground Leg: Connect a 10kΩ resistor from Pin 1 to Ground.
The Math:
$$V_{pin1} = 310V \times \left( \frac{10k}{1,200k + 10k} \right) \approx 2.56V$$Result: When the output hits ~310V, Pin 1 will match the 2.5V on Pin 2. If the output drops (due to load), the voltage on Pin 1 drops, and the TL494 automatically increases the pulse width to compensate.
2. Stabilize the Loop (Compensation)
Switching power supplies can "hunt" or oscillate if the feedback is too fast.
The Fix: Connect a 100nF capacitor and a 47kΩ resistor in series between Pin 3 (Feedback) and Pin 2. This "slows down" the error amplifier just enough to keep the voltage steady without jittering.
3. Diode Rectifier and Smoothing Capacitor
In the current circuit, I have used a Ferrite core and so the output at J2 is high-frequency AC. This output must be converted to to High-Voltage DC (around 310V–340V DC) first. To do this we have to add a bridge rectifier and a filter capacitor.
Bridge Rectifier: We need to add a full-bridge rectifier to the output of TR1. We cannot use standard 1N4007 diodes here; you must use Ultra-Fast Recovery Diodes (like the UF4007 or HER308) because standard diodes are too slow for high-frequency switching and will overheat. I have used UF5406 diodes here.
Filter Capacitor: Add a high-voltage electrolytic capacitor after the rectifier (e.g., 47µF to 100µF, 450V). This will give you a stable DC "bus."
The Feedback Voltage regulation: This has already been discussed above. Without feedback, the voltage at your output will skyrocket when there is no load (possibly over 600V), which will blow your capacitors. The output voltage from this high DC voltage is feedback to Pin 1 (1IN+) of the TL494.
4. The Snubber Circuit: Protection and Stability
I have added the RC Snubber (the $10\Omega$ resistor and $10\text{nF}$ capacitor) which serves as a critical safety net for your MOSFETs.
1. Why it is Mandatory
When the MOSFETs (Q3–Q6) switch off, the magnetic field in the transformer primary collapses. This creates a massive inductive spike (back-EMF) that can exceed the 55V limit of the IRFZ44N. The snubber "snubs" or absorbs this high-voltage energy, protecting the MOSFETs from internal breakdown and heat failure.
2. Improving Output Stability
As you noticed during your testing, the DC output can sometimes "bounce" or oscillate. The snubber helps by:
Damping Ringing: It stops high-frequency oscillations (noise) on the primary side that can confuse the TL494's feedback loop.
Cleaning the Waveform: It turns sharp, chaotic spikes into smooth curves, making the simulation math easier for Proteus and the physical circuit more efficient.
3. Component Selection
The Capacitor ($10\text{nF}$): Acts as a high-frequency bypass, capturing the energy of the spike. In the real build, this should be a high-voltage ceramic or film capacitor ($100\text{V}$ or higher).
The Resistor ($10\Omega$): Dissipates the captured energy as a tiny amount of heat. For the physical build, use a 2W resistor to handle the constant switching stress.
You can use the snubber circuit calculator to calculate the values or R and C values.
The following shows circuit diagram of the above-mentioned modification.
2nd stage of TL494 Modified Sine Wave Inverter Design
1. Dead Time Control (DTC)
To convert the TL494 push-pull square wave inverter into modified sine wave inverter we need to adjust the dead time. In the current TL494 push-pull inverter, square wave is produced, whose frequency is set by the capacitor CT pin and resistor pin RT (above 20khz), and each side is of the MOSFETs are ON for 50% of the cycle. A Modified Sine Wave introduces a "dead zone" where both sides are "off," creating a stepped waveform (0V → +Peak → 0V → -Peak → 0V). This is much easier on sensitive electronics than a raw square wave and is achieved by adjusting the Dead-Time Control (DTC) on Pin 4.
Currently, square wave is produced because the Pin 4 is tied to ground, which gives maximum "on" time (effectively a square wave). To get a modified sine wave, we need to apply a small positive voltage to Pin 4. So, for modified sine wave generation we need to remove the direct connection from Pin 4 to Ground. Then create a voltage divider between the 5V Reference (Pin 14) and Ground (Pin 7) and connect the junction to the pin 4. A true modified sine wave usually has the MOSFETs "on" for about 70% to 75% of each half-cycle. To achieve this, we can use a 10k potentiometer or a divider where Pin 4 receives roughly 0.5V to 1.5V. Increasing this voltage narrows the pulse width, creating the "stepped" gap of the modified sine wave.
2. Frequency Adjustment
Also, a modified sine wave inverters usually operate at standard grid frequencies (50Hz or 60Hz). But the current timing components are C1 (15nF) and RV1 (2.2k). For a 50Hz output in push-pull mode, the TL494 internal oscillator needs to run at 100Hz. The freqeuncy is determined by the following equation:
$f_{osc} = \frac{1.1}{R_T \times C_T}$
To hit 100Hz with a 15nF cap, you would need a much larger resistor (around 700kΩ). So, it is better to swap C1 for a 1µF electrolytic or film capacitor to keep the resistor values in a more manageable range (around 10kΩ).
5. Adding the "Modified Sine" Stage (H-Bridge)
To get 50Hz AC out of your 310V DC bus, you need a second stage.
The H-Bridge: You need four more MOSFETs (rated for 400V or higher, like the IRF840) arranged in an "H" pattern.
The Controller: You can use another TL494 or an Arduino set to 50Hz to drive this H-bridge. By using the Pin 4 (Dead-Time Control) trick we discussed earlier on this second TL494, you create the "Modified Sine" steps.
6. Snubber Circuits (Load Protection)
In the 2nd stage (AC output), the RC snubber is placed directly across the Live and Neutral terminals. Its primary job is to protect the H-bridge from the "kickback" of real-world appliances.
Protects the H-Bridge: When driving inductive loads (fans, motors, or transformers), it absorbs high-voltage spikes caused by the 50Hz switching.
Filters Noise: It rounds off the sharp edges of the Modified Sine Wave, reducing electromagnetic interference (EMI) and preventing "ghosting" in electronic devices.
Key Components: * Resistor: $100\Omega$ to $220\Omega$ (3W–5W).
Capacitor: $10\text{nF}$ to $47\text{nF}$ (Must be rated for 630V).
That's it. This is how I converted my TL494 simple square wave inverter circuit design into modified sine wave inverter.
The following shows the complete circuit diagram showing both the 1st and 2nd stage of the TL494 modified sine wave inverter design.
You can watch the video which explains how the circuit works.
Next, I will make a simulation video on this.
You can download the proteus project file from the link below:
Download TL494 Modified Sine Wave Inverter Proteus Project/Schematic/Code
Related Useful links:
